The Indispensability of Dispensable Amino Acids In Feeding Pigs

Optimizing amino acid nutrition is central to efficient pork production. While feeding programs correctly prioritize the indispensable amino acid content and digestibility of feeds, the importance of dispensable amino acids should not be overlooked in pigs. These amino acids serve as substrates for protein synthesis and as key regulators of intermediary metabolism, immune function, and gut integrity. Dispensable amino acids have long been assumed to be synthesized in sufficient quantities to meet the metabolic demands of growing pigs, as long as total crude protein intake exceeds estimated requirements. However, in low crude protein diets with increasing crystalline amino acid use, the amino acid composition of crude protein above the estimated requirement for indispensable amino acids and below the estimated requirement for crude protein should be considered. Indeed, differences in the amino acid composition of this fraction could contribute to the variation in growth performance observed across studies that evaluate feeding of low crude protein diets to pigs. The assumption that nitrogen supplied by each amino acid is equally effective for dispensable amino acid synthesis is likely not true, especially for the threonine, serine, glycine, and cysteine. Rodent studies suggest that about 85% of cysteine sulfinate, the first committed intermediate of cysteine oxidation, is converted to taurine, leaving only 15% available for transamination. Since taurine nitrogen is not recycled, cysteine oxidation to taurine represents an irreversible loss of nitrogen from the dispensable amino acid pool. Although this type of quantitative cysteine partitioning data in pigs is limited, these findings suggest that only 15% of nitrogen liberated from cysteine during its degradation can be used to synthesize other dispensable amino acids. This may be particularly relevant in the gut that tends to rely on dietary versus systemic amino acid supply for protein synthesis and other functions. Potential interactions between feed additives and amino acid metabolism should also be explored. Benzoic acid, an organic acid, is reported to increase growth performance and protein digestibility in nursery pigs and reduce manure ammonia emissions in finisher pigs. In pigs, benzoic acid is excreted mainly in urine as hippuric acid after conjugation to glycine in the liver. We showed that feeding benzoic acid reduces growth performance, feed efficiency, and plasma glycine concentrations in nursery pigs fed crude protein-deficient diets. Further, plasma hippuric acid, urine hippuric acid, urine hippuric acid excretion, and urine nitrogen excretion increase with benzoic acid supplementation in a dose-dependent manner. Better understanding of how dispensable amino acid-derived nitrogen is used in pigs will help us identify the circumstances in which dietary crude protein levels can be safely reduced without affecting animal performance.